[Haskell-cafe] Monad transformer responsibilities

Stephan Friedrichs deduktionstheorem at web.de
Mon Jun 8 07:06:30 EDT 2009

Henning Thielemann wrote:
> [...]
>>  - So you have to declare them near the test cases and they're orphan
>>    instances
>> The entire project doesn't issue a single warning when compiling with
>> -Wall *except* two orphan instances when building the test cases...
> However, I had sometimes the case, where a type from another library was
> part of my tests and thus I needed its Arbitrary instance. I could have
> defined instances for the foreign types, but they would have been orphan
> and I risk that the library author decides to add the instances later.

Hmm... maybe it is a good idea to aktivate the instance declaration with
a cabal flag? I've already got:

Flag Test
  Description:       Build a binary running test cases
  Default:           False

and I could easily add something like

if flag( Test )
  CPP-Options:       -D__TEST__
  Build-Depends:     QuickCheck >= 2 && < 3


data MyType = ...

#ifdef __TEST__
instance Arbitrary MyType where

A usage of cabal flags that strongly reminds me of Gentoo's useflags :)
However, this will result in a total mess with more than one such flag...



Früher hieß es ja: Ich denke, also bin ich.
Heute weiß man: Es geht auch so.

 - Dieter Nuhr

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